The present invention relates to a Faraday rotator and a method for preparing the same as well as a magneto-optical element in which the Faraday rotator is incorporated and an optical isolator as one of the magneto-optical elements.
In an optical circuit, which constitutes an optical communication system, there have been used a variety of magneto-optical elements such as an optical isolator, an optical circulator, an optical switch, a magnetic field sensor and an optical attenuator. Such a magneto-optical element comprises a Faraday rotator as an optical component and therefore, the operating ability of the magneto-optical element is changed due to the temperature of the environment in which the element is operated (hereunder referred to as xe2x80x9cuse environmentxe2x80x9d). For this reason, a temperature-control device is fitted to the magneto-optical element in the use environment whose temperature is severely changed, but the use of the control device is not necessary if the temperature-dependency of the Faraday""s rotational angle of the Faraday rotator is low.
As to the improvement of the Faraday rotator in the temperature-dependency, Japanese Patent No. 2,679,157 discloses the use of a terbium-bismuth-gallium iron garnet crystal. This crystalline body is grown by the flux method and therefore, it suffers from a problem such as low reproducibility and productivity and difficulty in processing.
On the other hand, it has been tried to prepare a garnet-epitaxial film according to the liquid phase epitaxy, but the usually available garnet crystals are those obtained by replacing or substituting Gd3Ga5O12 with Ca, Mg, Zr (an NOG substrate manufactured by Shin-Etsu Chemical Co., Ltd. and having a lattice constant of 12.496xc2x10.003 xc3x85) and Sm3Ga5O12 (an SGG substrate having a lattice constant of 12.439 xc3x85).
Accordingly, it is an object of the present invention to, in a broad sense, solve the foregoing problems associated with the conventional techniques and to, in a narrow sense, provide a Faraday rotator whose temperature-dependency in the Faraday""s rotational angle is low, a method for efficiently preparing the same, a magneto-optical element, which makes use of the rotator and whose quality and characteristic properties are less sensitive to a temperature change in the use environment and an optical isolator, at a low price.
The inventors of this invention have variously investigated any change in the Faraday""s rotational angle of the Faraday rotator due to temperature changes in the use environment, have found that the temperature-dependency of the Faraday""s rotational angle is adversely affected by the chemical species of ions, which occupy the positions 24c in the garnet crystal structure constituting the Faraday rotator and have thus completed the present invention.
According to a first aspect of the present invention, there is provided a Faraday rotator, which consists of a garnet crystal represented by the following compositional formula and having a lattice constant of 12.470xc2x10.013 xc3x85:
(Tb1xe2x88x92(a+b+c)LnaBibM1c)3(Fe1xe2x88x92dM2d)5O12xe2x80x83xe2x80x83I
In Formula I, Ln is an element selected from the group consisting of rare earth elements other than Tb; M1 represents an element selected from the group consisting of Ca, Mg and Sr; M2 is an element selected from the group consisting of Al, Ti, Si and Ge; and a to d are numerals satisfying the following relations: 0xe2x89xa6axe2x89xa60.5, 0 less than bxe2x89xa60.2, 0xe2x89xa6cxe2x89xa60.02 and 0xe2x89xa6dxe2x89xa60.1.
Among the foregoing Ln, preferred are, for instance, La, Pr, Nd, Gd, Dy, Ho, Yb, Lu and Tm. These elements have ionic radii different from one another and these elements can be incorporated into the crystal in an appropriate amount which falls within the range: 0xe2x89xa6axe2x89xa60.5 so that the lattice constant thereof falls within the range: 12.470xc2x10.013 xc3x85. If the rate b of Bi present exceeds 0.2, the lattice constant of the resulting crystal is beyond the desired range defined above. The elements Ca, Mg and Sr represented by M1 have a light absorption-inhibitory effect to thus improve the optical transmission and it is sufficient to use these elements in a trace amount (rate c) on the order of not more than 0.02. The elements Al, Ti, Si and Ge represented by M2 are replaced with Fe atoms in the garnet crystal. The element Al is involved in the lattice constant and saturated magnetic flux density of the crystal. The elements Ti, Si and Ge serve to prevent light absorption like the elements Ca, Mg and Sr, when the coexisting Fe ions are in the divalent state. If the rate d of these elements exceeds 0.1, the lattice constant of the resulting crystal is beyond the desired range defined above and the saturated magnetic flux density would exceed 1000 gauss (Gs).
As specific examples of the garnet crystals represented by Formula I, preferred be one represented by the following compositional formula: Tb2.48Bi0.52Fe5O12.
According to a second aspect of the present invention, there is provided a method for preparing a Faraday rotator, which comprises the step of growing a garnet crystal represented by Formula I and having a lattice constant of 12.470xc2x10.013 xc3x85 on a substituted or unsubstituted gadolinium-gallium-garnet crystalline substrate having a lattice constant of 12.472xc2x10.013 xc3x85 according to the liquid phase epitaxy.
More specifically, the method for preparing a Faraday rotator is the liquid phase epitaxy, which comprises the steps of, for instance, mixing garnet components such as a combination of Tb4O7, Bi2O3 and Fe2O3 with flux components such as a combination of B2O3 and PbO, melting the resulting mixture in a platinum crucible, and immersing a paramagnetic garnet crystalline substrate in the molten mixture while maintaining the temperature of the melt constant and rotating the substrate to thus grow the foregoing desired crystal on the substrate. The lattice constant of the paramagnetic garnet substrate is 12.472xc2x10.013 xc3x85 and therefore, it is possible to use a substituted crystalline substrate obtained by the substitution of a gadolinium-gallium-garnet crystal with Ca and/or Zr through addition thereof.
According to a third aspect of the present invention, there is provided a Faraday rotator, which consists of a garnet crystal represented by the following compositional formula II and having a lattice constant of 12.470xc2x10.013 xc3x85:
(Tb1xe2x88x92(a+b+c+e)LnaBibM1cEue)3(Fe1xe2x88x92dM2d)5O12xe2x80x83xe2x80x83II
In Formula II, Ln represents an element selected from the group consisting of rare earth elements other than Tb and Eu; M1 represents an element selected from the group consisting of Ca, Mg and Sr; M2 is an element selected from the group consisting of Al, Ti, Si and Ge; and a to e are numerals satisfying the following relations: 0xe2x89xa6axe2x89xa60.5, 0 less than bxe2x89xa60.2, 0xe2x89xa6cxe2x89xa60.02, 0xe2x89xa6dxe2x89xa60.1 and 0 less than exe2x89xa60.3.
The Faraday rotator according to the third aspect of the present invention is identical to the rotator according to the first aspect of the present invention except that a part of Tb present in the garnet crystal is substituted with Eu. In these Faraday rotators, the Faraday""s rotational angle varies as a quadratic curve with respect to the temperature, but Eu serves to control the temperature at which the quadratic curve has a peak. Thus, the peak temperature of the Faraday""s rotational angle can be controlled to a desired value (such as room temperature) by adjusting the e value to a level of not more than 0.3. In addition, Eu has an effect of inhibiting light absorption to thus improve the optical transmission of the rotator, like Ca, Mg and Sr. Therefore, if Eu is incorporated into the crystal to control the peak temperature of the Faraday""s rotational angle, the rate c of Ca, Mg and Sr represented by M1 can be reduced in proportion thereto.
As specific examples of garnet crystals represented by Formula II, preferred be crystals represented by the following compositional formulas: Tb2.42Eu0.06Bi0.52Fe5O12 and Tb2.42Eu0.06Bi0.52Fe4.95Al0.05O12.
The method for preparing a Faraday rotator according to a fourth aspect of the present invention comprises the step of growing a garnet crystal represented by Formula II and having a lattice constant of 12.470xc2x10.013 xc3x85 on a substituted or unsubstituted gadolinium-gallium-garnet crystalline substrate having a lattice constant of 12.472xc2x10.013 xc3x85 according to the liquid phase epitaxy.
A method for preparing this Faraday rotator is identical to that for preparing a Faraday rotator consisting of the garnet crystal represented by the foregoing Formula I except that Eu2O3 as a raw material is incorporated into the crystal.
A magneto-optical element according to a fifth aspect of the present invention is one selected from the group consisting of an optical isolator, an optical circulator, an optical switch, a magnetic field sensor and an optical attenuator and is characterized by comprising, as a component, the Faraday rotator according to the first or third aspect of the present invention.
A sixth aspect of the present invention relates to an optical isolator among the magneto-optical elements and comprises a polarizer and an analyzer whose planes of polarization are rotated with respect to each other at an angle of 90xc2x0 and the Faraday rotator according to the first or third aspect of the present invention arranged between the polarizer and the analyzer.
A seventh aspect of the present invention relates to a preferred embodiment of the sixth aspect of the present invention and is characterized in that the Faraday""s rotational angle of the Faraday rotator falls within the range of 36 to 44xc2x0.
Incidentally, these optical isolators each comprises a polarizer, a Faraday rotator and an analyzer which are arranged in this order as well as a magnetic body surrounding the Faraday rotator. They may be in the form of a single-step or multi-step type one. The polarizer and the analyzer have the same configuration and may be, for instance, a glass polarizing plate or a dielectric metal laminated film polarizer. Alternatively, an optical member such as a wavelength plate may be inserted into the isolator in addition to these components.
In the structure of the garnet crystal constituting the Faraday rotator according to the present invention, Bi ions and Tb ions as well as Eu ions are introduced into the 24c positions of the crystal. Thus, the Faraday""s rotational angle varies as a quadratic function of temperature as the temperature of the use environment varies and the temperature at which the Faraday""s rotational angle undergoing variation as a quadratic function of temperature has a peak can be controlled to a desired level by adjusting the amount of Eu to be incorporated into the crystal. For this reason, if the Faraday""s rotational angle is controlled in such a manner that the low temperature-dependent region thereof is located at a temperature in common use such as room temperature, the Faraday rotator of the present invention would provide a stable Faraday""s rotational angle even if the temperature of the use environment varies.
The method for preparing the Faraday rotator according to the present invention comprises the step of growing a garnet crystal having a lattice constant of 12.470xc2x10.013 xc3x85 and is characterized in that a substituted or unsubstituted gadolinium-gallium-garnet crystal having a lattice constant of 12.472xc2x10.013 xc3x85 is used as a crystalline substrate for liquid phase epitaxial growth. Accordingly, the resulting epitaxial crystalline layer is not susceptible to any crack because of high degree of crystal matching between the grown crystal and the crystalline substrate. Therefore, the Faraday rotator of the present invention can efficiently be produced in large quantities.
On the other hand, the magneto-optical element according to the present invention makes use of the Faraday rotator of the present invention whose Faraday""s rotational angle is less dependent on the temperature as a component of the element and therefore, it can provide stable magneto-optical characteristics even if it is used in an environment which undergoes a temperature change.
Moreover, the optical isolator of the present invention, which makes use of the Faraday rotator according to the present invention would provide a stable extinction ratio even if it is used in an environment whose temperature varies.
Furthermore, in the optical isolator of the present invention, the Faraday""s rotational angle of the Faraday rotator of the present invention, which is incorporated therein, is controlled to the range of from 36 to 44 deg. Therefore, the plane of polarization of the polarized light rays which can transmit through the polarizer and the analyzer or can be cut off by these elements is not consistent with or perpendicular to the direction of the plane of polarization of the polarizer and the analyzer. For this reason, the extinction ratio and the insertion loss of the resulting optical isolator are deteriorated to some extent. However, the change in the Faraday""s rotational angle with temperature is considerably reduced as compared with that expected when the Faraday""s rotational angle is set at 45xc2x0 and therefore, the variation of the extinction ratio within the same temperature range is also greatly reduced. The use of the Faraday""s rotational angle of less than 36xc2x0 is not preferred, since the deterioration of the insertion loss of the resulting optical isolator is equal to 1 dB.